Numerous applications exist that require the formation of polymeric objects with a surface composition distinct from that of the core material. For example, ophthalmic lenses can be tinted to create sunglasses or photochromic lenses. Tinting involves the absorption of dye into the surface layer of the lens. The current practice is to first create a clear lens, often by grinding and polishing a lens blank into the precise shape/contour. In certain instances, injection molding can be employed to create the prescription. The finished lens is then dipped in a dye-containing solution at elevated temperatures (e.g., in near-boiling-temperature aqueous or organic solutions). High temperatures are needed to soften and dilate the lens material to allow penetration of the dye molecules into the tight plastic network constituting the lens. This tinting (dye absorption or uptake) process is slow, even under such severe conditions. The use of high temperatures can cause dye degradation (thus necessitating frequent bath replenishment) and often leads to lens warpage. Photochromic dyes are known for their tendency toward thermal degradation, making photochromic lens manufacture a difficult task. Insufficient uptake of the photochromic dye often results and this is a primary reason such lenses often do not turn sufficiently dark when exposed to sunlight. Also, since dye molecules are quite large, the crosslinks in the plastic network must be fairly loose to allow for the penetration of the dye. Additionally, the choice of dye is greatly limited by the fact that the process requires water-soluble dyes that will also be dispersible in an organic resin matrix.
Certain objects, such as eyeglasses, also often require scratch-resistant surface coatings. Presently, finished lenses can be coated with a scratch-resistant material in a dipping tank, and the scratch-resistant material is then cured. Alternatively, spin coating and spray coating can be used as deposition means. Regardless of the method of application, the scratch- or abrasion-resistant coating forms a separate layer, distinct from the existing lens. Physical interactions are relied upon to ensure (often imperfect) adhesion between the coating and the lens core, and delamination of the coating often occurs. Thus, there is a need to prepare “coated” lenses where the coating and the lens core actually form a continuous, monolithic, integrated structure. Delamination of the “coating” will therefore no longer be an issue for such lenses.
Contact lens technologies have also evolved significantly since the introduction of the lenses. Small, pre-cured, “buttons” were ground and polished to create the needed prescription. Alternatively, polymer precursors can be used to fill mold cavities, which are then cured to form the finished lenses. Here, similar to ophthalmic lenses, shrinkage accompanying cure must be accounted for in the mold design. In either case, the finished lenses can subsequently be tinted by straightforward uptake of a dye, with the accompanying problems discussed above with respect to ophthalmic lenses, or by “printing” a pattern using a variety of techniques. The printing process results in lenses mimicking the pigment distribution of a human iris. Tinting or printing (transfer printing, ink jet pattern deposition, or screen-printing) are all dictated by the dye uptake rate and strength of dye adhesion to the lens material.
In contrast to ophthalmic lenses, contact lenses must possess two additional properties. One is high oxygen permeability. The second is biocompatibility. Oxygen permeability has been found to be relatively high in soft, rubbery materials such as silicones. Silicones tend to exhibit oxygen affinity and rapid transport. Permeability is a product of diffusivity and solubility at steady state. Oxygen molecules in soft materials tend to exhibit at least high diffusivity if not high solubility as well. Block co-polymers of controlled morphology have been used to achieve high flux and dimensional stability. Morphology control is required to ensure optical transparency. Highly crosslinked silicones can also promote dimensional stability, which is necessary for precision of prescription. However, most polymers with high oxygen permeabilities do not exhibit optimal tissue biocompatibility. A certain degree of hydrophilicity is needed to give a “hydrogel-like” surface layer to ensure comfort for the lens wearer. Surface modification schemes, such as oxidation and plasma treatment have been employed to achieve some level of wettability. Such processes, however, add cost to the manufacture. Creation of a surface layer also implies possible adhesion issues of the layer to the core lens. It would be preferable to have a monolithic object with surface composition differing from the core in a controlled manner.